The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A FMCW radar method involves the use of a radar system of a motor vehicle that emits and receives radar waves, and in which a distance to an object with respect to the motor vehicle can be determined based on a frequency shift between the emitted and received radar waves and in which a speed of an object may be determined based on the phase positions of received radar waves.
A method of this kind and a radar system such as this are known from DE 196 10 970 A1. Very generally, the frequency of emitted radar waves is periodically varied in time in a FMCW radar system according to a predetermined pattern. Radar waves reflected on an object have traveled twice the distance to the object when they are received by the radar system and meet again in the radar system with a time delay that is proportional thereto.
Since the frequency of the radar waves to be emitted has changed during this time delay, the emitted signals and received signals propagating within the radar system have a frequency difference d_f(r) at a specific time point, which is dependent on the object distance r and the type of emission frequency variation. With linear time variation and object at rest relative to the radar system, d_f(r) is directly proportional to the object distance. During a relative movement with relative speed v, there occurs in addition a speed-dependent Doppler shift of the frequency, whose sign is dependent on the direction of the speed and on the sign of the emission frequency variation.
In order to determine the frequency difference, the emitted signal is usually mixed with the received signal and forms an intermediate frequency signal, which has as a consequence a spectral signal portion within the magnitude of the frequency difference and further portions with higher frequencies. Through the use of low pass filtering, the portions with higher frequencies are separated and the remaining signal, in which the runtime-dependent and speed-dependent frequency shifts are reproduced, is spectrally analyzed.
According to DE 196 10 970 A1, a frequency value (spectral line) is essentially obtained for the periodic amplification of the emission frequency in the sum of the distance-dependent and speed-dependent frequency shifts, while a frequency value in the difference of the mentioned frequency shifts is obtained for periodic reductions of the emission frequency. By forming the mean value and difference of these frequency values, the values of the distance-dependent and speed-dependent frequency shifts can be individually determined. The distances and the speeds are thus ultimately determined in this evaluation from the frequency of the intermediate frequency signal. This requires, however, a clear allocation of spectral lines and objects, which, in case of several reflecting objects at similar distances, is not readily ascertainable.
In order to determine the distances and speeds of each individual object when there are several objects, the initially mentioned DE 196 10 970 A1 proposes to derive the distances from the frequency of the intermediate frequency signal and the speeds from the phase [position] information of the intermediate frequency signal. According to DE 196 10 970 A1, the argument of the intermediate frequency signal, in other words its phase, thus contains especially a distance-dependent term. Since the distance changes slightly between two periods of the emission frequency variation in case of a relative movement between radar systems and object, the distance-dependent term in the phase position of the intermediate frequency signal also changes. From the extent of change of the phase [position] during at least two periods of the emission frequency variation should be deduced the relative speed according to DE 196 10 970 A1. The phase [position] information is respectively obtained from a phase position of a Fourier transform of the intermediate frequency signal. In order to obtain two phase values, two periods of the variation of the emitted signal must therefore be run through for each object and a Fourier transform must be carried out for each period.